Metallic Twin Grain Boundaries Embedded in MoSe₂ Monolayers Grown by Molecular Beam Epitaxy

Twin grain boundaries in MoSe₂ are metallic and undergo a metal to insulator Peierls transition at low temperature. Growth of MoSe₂ by molecular beam epitaxy results in the spontaneous formation of a high density of these twin grain boundaries, likely as a mechanism to incorporate Se deficiency in the film. Using scanning tunneling microscopy, we study the grain boundary network that is formed in homoepitaxially grown MoSe₂ and for MoSe₂ grown heteroepitaxially on MoS₂ and HOPG substrates. No statistically relevant variation of the grain boundary concentration has been found for the different substrates, indicating that the grain boundary formation is substrate independent and depends mainly on the growth conditions. Twin grain boundaries exhibit three crystallographically identical orientations, and thus they form an intersecting network. Different intersection geometries are identified that imply varying defect configurations. These intersection points act as preferential nucleation sites for vapor-deposited metal atoms, which we demonstrate on the example of selective gold cluster formation at grain boundary intersections. Scanning tunneling spectroscopy shows a band gap narrowing of MoSe₂ in the immediate vicinity of the metallic grain boundary, which may be associated with lattice strain induced at the grain boundary. Tunneling noise spectra taken over the grain boundaries indicate random telegraphic noise, suggestive of pinning/depinning behavior of conductive channels in the metallic grain boundaries or their intersection points. Finally, indications for incommensurate and commensurate Peierls-driven charge density wave formation were observed in microprobe transport measurements at 205 and 227 K, respectively.